In 2387, a star in close proximity to Romulus went supernova and threatened the galaxy. To stop the threat, AmbassadorSpock used red matter to create a black hole which consumed the star's energy. Before he could successfully carry out his mission, however, the supernova reached Romulus, destroying the planet and all of its inhabitants. Among those killed were the wife and unborn child of Nero, who swore vengeance against Spock, Vulcan, and the Federation for letting his world die. While Nero confronted Spock, both of their ships were pulled into the black hole which had consumed the star, sending them back in time and resulting in the creation of the alternate reality. (Star Trek)
A star near the Romulus system just before going supernova
In the comic book series Star Trek: Countdown, this star and its system were named "Hobus." The comic explained that the star's galactic threat came from its unusual ability to transform anything it contacted into energy, which it then consumed, increasing its power and causing it to expand farther and faster throughout the galaxy. In Star Trek Online, a scandal occurred several years later. It involved rumors that the Vulcan Science Academy knew about the instability of the star but did nothing until Spock attempted to save Romulus with the Jellyfish. The scandal resulted in several resignations and sowed seeds of distrust towards the Vulcans.
It is also confirmed by the Featured Episodes "Cloaked Intentions" that Remus was also destroyed, however the Romulans and Remans still continue their prejudices towards each other. It is also revealed that the supernova was not a natural occurrence, but was engineered by then-PraetorTaris, who did so under orders from the Iconians.